January 26, 2008

I was going to write about mystery cervicals of the Cloverly Formation, but that requires knowing something about juvenile vertebrae and Pleurocoelus, so I decided to write about Pleurocoelus, but that still requires knowing something about juvenile vertebrae. So I’m writing this tutorial to lay the groundwork for more goodness to come.

Sutures

Vertebrae do not spring forth fully formed, like Athena from the mind of Zeus. They grow from bits, and the bits come together at different times in development. The bits themselves start out as anlagen–bone precursors–made of cartilage, and these anlagen start to ossify–turn into bone–at one or more ossification centers. From those centers, the bone grows outward and replaces the cartilage like some kind of science fiction blob monster taking over its host. As the bone replaces the cartilage, the contacts between different bony elements are sometimes left behind as sutures, like the sutures in the bones of your skull. Once the replacement of cartilage by bone is complete, most of the new bone growth happens at the suture margins. This is easy to demonstrate experimentally: if you cut out the suture and glue together the bones on either side, the combined element will not grow to the normal length. Premature suture closure can be a big problem, if the bones that are now fused (say, skull bones) can’t grow fast enough to keep up with whatever is inside (say, a brain). And many of the sutures between skull bones stay open even into old age, at least in humans and other mammals (birds are another story), because sutures also serve another purpose, which is to help the skull respond to mechanical stress without cracking like an egg. However, the eventual fate of most sutures is to close as the bones on either side finally fuse. Some old folks do eventually fuse up most or all of the sutures in their skulls.

If you look across the whole skeleton, the closures of different sutures–in the vertebrae, in the long bones, in the limb girdles, in the skull–are spread out through time. This is nice if you want to determine the age at which something–or more often, someone–died (forensic anthropologists get a lot of mileage out of this). But it can also be a pain, because it means that some things are hardly ever found intact. Sauropod skulls are particularly prone to exploding. There are complete, articulated sauropod skeletons for which the skull is either scattered all over the place or simply gone.

How Vertebrae Form

Vertebrae form as several distinct pieces. The centrum starts from paired ossification centers on either side of the cartilaginous notochord (see Tutorials 1 and 2 if you need to brush up on vertebral anatomy). The neural arches also start out as left-and-right paired elements, each of which forms one half of the arch over the spinal cord. The left and right components of the centrum fuse very early in development, and the left and right halves of the neural arch come together later. All of these events can and do fail on occasion. The anlagen may form asymmetrically or not at all, parts may not ossify, and left-right halves can fail to fuse. The best known pathology associated with vertebral development in humans is spina bifida, in which the two halves of the arch and spine fail to unite. The growing spinal cord can stick out through the hole and cause all kinds of problems.

I’ll not dwell long on the embryology of vertebrae; there are whole textbooks full of that stuff if you’re curious and some good websites, too. In sauropods, in all of the embryos that have been discovered to date the vertebrae are not yet ossified, so there’s nothing to talk about.

Usually in juvenile dinosaurs you find the centrum as a single unit and the neural arch and spine as another (the exception is the atlas, the first cervical vertebra right behind the head, which is so weird that it will have to be dealt with in a separate post). The centrum and neural arch complex come together at the neurocentral sutures, a pair of zipper-like tracts of rough bone (and, in life, cartilage) on either side of the neural canal. These sutures stay open for a long time, usually until the dinos are around half-grown.

Here are a couple of cervical centra from a juvenile Apatosaurus (in this and other photos in this post, click on each picture to see the unlabeled version). They are facing left, and the giant depressions in the sides are the pneumatic cavities. The centra (C3 and C4 if you’re curious) are propped up on their oversized parapophyses, which are typical for Apatosaurus.

Here’s C6 from the same specimen, in anterior view. For this shot I put a thin sheet of foam over the centrum so that I could put the neural arch in anatomical position and see how the whole vert (minus cervical ribs, which form separately and fuse even later) would have looked.

Same vert in left lateral view. Compare to the first picture above to see how the rough patches on top of the centra match the shape of the corresponding patches on the neural arch.

Here are C7 and C8 in right lateral view. The neural arches and centra are preserved together, but the sutures are still plainly visible. Had this individual lived longer, the neural arches and centra would have grown together, and the sutures would have been gradually erased by bone remodeling.

Stages of Neurocentral Suture Fusion

Because neurocentral sutures close over time, they can tell us something about how mature an individual dinosaur was when it died. Each vertebra goes through several stages as the sutures close. It’s a continuous process and you could divide it into any number of stages depending on how picky you want to be, but for this case I’m going to use four:

completely unfused, with the neural arch and centrum as separate pieces that come apart after death

partly fused, in which the neural arch and centrum are starting to grow together but the suture is still clearly visible on the surface

mostly fused, with the neural arch and spine co-ossified, but with a suture still visible as a small line or scar on the surface

fully fused, with no visible trace of the suture, which has been obliterated by bone remodeling

Forensic anthropologists usually divide vertebrae into three bins based on neurocentral suture closure: unfused (1 above), fusing (2 and 3), and fused and obliterated (4). The problem is that you can’t really tell 2 and 3 apart based on external examination; a vertebra with a visible suture might be pretty well fused or it might be held together by only a handful of tiny bars of bone. X-rays or histological sectioning can solve the problem, but usually it’s not warranted; those tests cost time and money and the human skeleton has many other and better indicators of age.

For paleontologists the problem is even worse, because we can’t tell 1 apart from 2 or 3. A vertebra with a visible suture line might not be fused at all; the centrum and arch might just be preserved in full articulation. The c7 and C8 shown above could be in 1, 2, or 3; without cutting them up it’s probably impossible to say. I doubt that even the current generation of medical CT scanners could resolve the sutures–which are convoluted in all three dimensions and probably packed with dense matrix–well enough. If there is no trace of a suture we say that it is closed or fused, and if the suture is visible (or if the arch and spine are preserved as separate pieces, as in C6 above) we say that it is open or unfused.

Actually there is a step 3.5 in the list above, a partly obliterated suture, in which the suture line is visible along part of its length but obliterated elsewhere. These don’t turn up very often–that is, every vertebra goes through a stage like that, but it is evidently pretty brief compared to the other stages because you don’t often come across vertebrae in this condition. Brochu (1996) showed a couple in croc vertebrae, but I’ve never seen one in a sauropod.

There is a final complication, which is that fusion of the neurocentral sutures usually proceeds along the vertebral column like a wave. In some tetrapods in starts in the neck and goes to the tail; in some it goes from tail to neck; in some it starts in the middle and goes in both directions; and in some fusion starts in more than one place. A little work has been done on this in sauropods, but I’ll save that for another post. If you’d like to read up on it in the meantime, see Brochu (1996) and Irmis (2007).

The Point (at last!)

The upshot of all of this is that if you find a sauropod centrum with no arch, or vice versa, you can be sure that the animal was not fully mature. Centra are pretty close to being cylindrical, which is a good shape for surviving the ravages of taphonomy. Neural spines are not, and they fragment pretty easily. There are a lot more juvenile sauropod centra with no arches, both in the ground and in museums, than there are arches with no centra, although I have seen a couple of the latter so they do exist.

Whew! If you made it this far, thanks for sticking around for the long anatomy slog. It’s all groundwork for talking about baby sauropod bits, so your diligence will be rewarded. Stay tuned, true believers.

———

All the vertebrae shown in this post are cervicals of CM 555, from the Carnegie Museum in Pittsburgh.

January 19, 2008

Seeing the photograph in the last post of the Mamenchisaurus hochuanensis cast at the Field Museum in Chicago reminded me of a picture I’ve been meaning to post for a while. M.hoch, as I like to call it (we’re on familiar terms) is known primarily from its type specimen CCG V 20401, which was nicely described and figured by Young and Zhao in 1972. There are several pretty good quality casts of this specimen around the world: I first saw one in the car-park of the Copenhagen Geological Museum, and Chicago was the third time I saw it (and by far the best due to the excellent sight-lines from the balcony, lighting and help from the museum staff) .

The second time I saw a cast of this specimen it was actually the same one that I’d seen in Copenhagen: it was owned by the Homogea Museum in Trzic, Slovenia, and its loan to Copenhagen had expired. By happy coincidence my day-job took me to Slovenia, only 40 km or so from Trzic, so on a spare day I took a taxi to the museum where I was shown to the M.hoch cast.

Remember that bit in The Hitch-Hiker’s Guide to the Galaxy where Arthur Dent is told that the plans for demolishing his house have been on display at the council office for six months? In fact, let me quote:

“The notice was posted at the office, sir.”
“Your ‘office’ was in a basement. I had to look all over the building just to find it.”
“That’s where the office is located!”
“It was dark.”
“The lights were out!”
“So were the stairs.”
“But still, you found the notice, sir?”
“Oh, yes. It was quite ‘clearly’ posted in a locked filing cabinet in a disused lavatory with a sign on the door saying ‘Beware of the leopard.'”

That’s how I felt when I saw the Mamenchisaurus cast:

Yes, it’s in a basement. Yes, the lights were out (though, to be fair, the stairs were not). Yes, the basement is flooded (a trick that Douglas Adams missed) and for good measure the light you can see there, a portable floodlight, is powered by an extension lead that runs through the flooding. However, they didn’t have a filing cabinet big enough, so it was at least on display in the basement.

The good news is that I was able, from bits and pieces in the corner of the basement, to assemble a scaffold from which I could view the elevated cervicals:

As you can see, it consists of a trolley frame with a piece of decomposing and warped chipboard on top, surmounted by a stepladder. Unfortunately, this is a pretty tall dinosaur and those cervicals are a good 4 m off the ground, so the only way I could see the dorsal surface was by perching right on the top rung of the ladder. I do have photographic evidence, taken by a workman who was doing something mysterious in the corner of the basement, although it’s not great quality — about as good as the typical Loch Ness Monster photo:

Yes, that’s me, risking life and limb in the cause of sauropod vertebrae.

Sadly the result of all this was not very useful: the very poor lighting meant that the photos I took are low on detail, difficult to interpret, and of little scientific value. On the positive side, it was later that same month that I was in Chicago to see the better cast of the same animal, so I got all the photos I needed in the end.

January 17, 2008

In the last post, an astute commenter asked about Hudiesaurus: “A first dorsal 550 mm–isn’t that in Argentinosaurus territory?”

Well, let’s find out.

Hudiesaurus sinojapanorum was described by Dong (1997) based on a partial skeleton from the Kalazha Formation in China. The holotype, IVPP V 11120, is an anterior dorsal vertebra. Referred elements include a nearly complete forelimb, supposedly from a smaller individual of the same taxon.

I don’t actually have a copy of Dong (1997), but I do have Glut (2000), which contains a pretty good summary and a couple of pictures. Here’s the holotype dorsal in posterior view, after Glut (2000), after Dong (1997).

According to Glut, the scale bar is 15 cm–I added the “15 cm” in the image above for ease of use. Presumably ‘sp’ is ‘spine’, ‘po’ is ‘postzygapophysis’, ‘dp’ is ‘diapophysis’, and ‘ce’ is ‘centrum’. Glut says:

As measured by Dong (1997), the holotype dorsal centrum of H. sinojapanorum has a length of 42 cm, 1.5 times longer than the comparable element in the 22-meter (about 75-feet) long mounted holotype skeleton of Mamenchisaurus hochuanensis, which was the largest sauropod known from China when it was described. From these dimensions, Dong (1997) estimated the total length of H. sinojapanorum to be about 30 meters (more than 100 feet) in length.

IF the scaled figure (from Glut [from Dong]) is accurate, and if the 42 cm length (reported by Glut [reported by Dong]) is accurate–and the uncertainties involved cannot be ignored, especially after the last post–then the vertebra has the following dimensions:

Centrum length: 42 cm

Cotyle height: 39 cm

Cotyle breadth: 42 cm

Total height: 78 cm

None of these are close to 550 mm, so there’s no telling where that measurement came from or what it refers to.

UPDATE: please read comment #4 below, by Mickey Mortimer, which sets the measurement record straight. And invalidates the specific numbers used in the rest of the post, but not the overall point.

Fortunately for us, the dimensions of the vertebrae of Mamenchisaurus hochuanensis are available for free in English, along with the rest of the description (Young and Zhao 1972), courtesy of the wonderful Polyglot Paleontologist site. Here’s the paper. Let’s do some comparin’.

D1 of M. hochuanensis has a centrum length of 25 cm, cotyle height of34 cm, cotyle width of 17 cm, and total height of 64 cm. The fact that the centrum is twice as tall as wide is almost certainly an artifact of the lateral compression that affects the whole vertebral column to some extent. Let’s say for the sake of argument that the cotyle was originally circular and 25 cm in diameter. The holotype of H. sinojapanorum is 68% longer, 60% larger in diameter, and 22% taller. So if the vertebra is actually D1 and if H. sinojapanorum was built like M. hochuanensis (be sure to keep your If Counter updated), it might have been anywhere from 27-37 meters long (89-121 feet).

A couple of points before we go on. First, that’s pretty big, but it’s also a huge range. At the low end, it’s no bigger than Diplodocus; at the high end, it’s one of the longest sauropods on record. So that math suggests that it was a big sauropod but doesn’t help us pin down how big it was. Second, you have to keep in mind that Mamenchisaurus hochuanensis is basically a ridiculous neck attached to an unremarkable body (at least in terms of size). If you ignore the neck, the animal was about the same size as Cetiosaurus or Haplocanthosaurus–about 75% the size of the well-known specimens of Apatosaurus, Diplodocus, and Camarasaurus, and no where near the size of Brachiosaurus (despite having a longer neck). It’s basically a weiner dog, with most of the weiner out in the neck. Which is how Hudiesaurus could be 22% bigger and still be about the same size as Diplodocus. Even if Hudiesaurus was 60% bigger than M. hochuanensis, it would still not be in Argentinosaurus range in anything but length.

Mike (white shirt on lift) with M. hochuanensis at the Field Museum. It is worth remembering that he would need the same lift at about the same height to work on the posterior cervicals of Brachiosaurus!

Also, assigning serial positions to isolated vertebrae is tough. What if Dong was off by a single position, and the holotype vert is actually the most posterior cervical? True, it doesn’t have fused cervical ribs, but cervical ribs can fuse pretty late in ontogeny. Furthermore, rib identities can get a little wonky in the cervico-dorsal transition. Sometimes you have a nice, well-behaved, fully-fused rib on the last cervical, and a nice, well-behaved, long mobile rib on the first dorsal, but sometimes there is a godawful Frankenstein rib that doesn’t fit neatly into either category. Anyway, we’re just playing “what if” here. Nobody should take this as gospel.

The last cervical of M. hochuanensis has a centrum length of 32.5 cm, an average cotyle diamter of 29 cm, and a total height of 66 cm. If the holotype of Hudiesaurus actually corresponds to this vert instead of D1, then it is 29% longer, 38% larger in diameter, and 18% taller. In other words, no bigger than Diplodocus. In which case, the articulated forelimb might belong with the dorsal vertebra after all. Although it was found more than a kilometer away from the vertebra, so the case for it’s referral to the same taxon is not strong. At all.

But that’s not all! Not all Chinese sauropods were hellaciously long-necked, a point made by Mickey Mortimer in his DML post on Hudiesaurus. Abrosaurus ha 13 cervicals, rather than 19 like M. hochuanensis, and its cervicals are only about a third longer than its dorsals (upshot: in neck-to-body proportions, it was built like Camarasaurus). If Hudiesaurus was built like a giant Abrosaurus, it might have approximated a large individual of Camarasaurus in both body size and neck length.

A final amusing point. Glut (2000, p. 235) includes a photograph of the articulated forelimb of Hudiesaurus on display at Dinofest ’98 in Philadelphia. Next to the forelimb is a string of 4 articulated dorsal vertebrae. What is this? The original paper only mentions one dorsal vertebra, so where did the other three come from? Sadly, they came out of the same mold as the first–if you look carefully at the photo, you can see that the exhibitors simply made four casts of the same vertebra and strung ’em together to look like something more complete.

It’s not uncommon to clone adjacent vertebrae to fill out mounted skeletons. Heck, the T. rex skeleton in the Valley Life Sciences Building at Berkeley (my old digs) has a block of 5 identical dorsals, and its caudals come in identical pairs all the way down the tail (easy to see even in the small photos here). But that’s a different case. T. rex is known from complete remains, and the cloning was only done to fill out a skeleton that was already mostly there. The exhibition of the cloned Hudiesaurus vertebrae bothers me, because it implies to observers that the animal is better known than it actually is. I wasn’t at Dinofest ’98 so I can’t tell you for certain that there wasn’t a sign right there that said, “Warning: this animal is based on one vertebra that we have cloned to show you what a string of them would look like!” but I seriously doubt that there was any indication at all (that’s no reflection on Dong; he probably had nothing to do with the choices made by the exhibitors).

Here’s the take-home message:

If you see an eyebrow-raising number tossed out regarding a giant dinosaur, don’t surrender your credulity until you or someone you trust have tracked down the sources. And anytime you see mounted material, it’s perfectly fair to ask how much of it is real.

January 16, 2008

Futalognkosaurusdukei, just described last year, may be the most complete giant dinosaur ever discovered. Maybe. Depends on what you compare it to, and it also depends on how it’s measured. It’s hard to say right now because only one short paper on it has been published to date (Calvo et al. 2007), and it only includes one figure of the beast. Let’s take a look.

Figure 2 from Calvo et al. (2007)

Now, according to the skeletal reconstruction and the 2 meter scale bar, the total length of the cervical series is 10.6 meters, the dorsal column is 4.85 meters long, and the sacrum is 1.75 meters long. By way of comparison, the complete cervical series of the HM SII skeleton of Brachiosaurus brancai is 8.5 meters (or would be, with the two missing cervicals included), and the really complete, articulated neck of Mamenchisaurus hochuanensis is 9.5 meters. So Futalognkosaurus indeed looks like a whopper.

But hang on a second. The figure also includes scale photos of several of the elements. It’s a cinch to use those scale bars to figure the size of the photographed elements, and compare the measurements obtained that way to the size of the same elements in the skeletal reconstruction. I made all the measurements using the ruler tool in Photoshop, and here’s what I got (feel free to do your own and compare). In each line, the first number is the one I get from the scaled skeletal reconstruction, the second is the number I get from the scaled photograph, and in parentheses is the second as a percentage of the first.

C2 is 18 cm… or 29 cm (161%)

C6 is 79 cm… or 46 cm (58%)

C10 is 94 cm… or 89 cm (95%)

C14 is 94 cm… or 50 cm (53%)

Caudal 1 is 92 cm tall… or 72 cm tall (78%)

Pubis is 149 cm … or 141 cm (95%)

Now, it’s interesting that except for C2, which is small and hard to draw right (trust me), all of the measurements based on photos are smaller than the measurements based on the skeletal reconstruction.

The only measurement given in the paper–other the total length of the animal, estimated at 32-34 meters (105-111 feet)–is the length of the pubis, which is actually 137 cm, which is 97% of the size indicated by the scaled photo and 92% of the size indicated by the skeletal reconstruction.

The story gets more interesting. As you can see from the numbers I slapped on the illustration, the reconstruction includes 14 cervicals, 11 dorsals, 6 sacrals, and 1 caudal. But in the character matrix at the end of the paper, Futalognkosaurus is coded as having 14 cervicals, 10 dorsals, and 6 sacrals. So either the skeletal reconstruction has an extra vertebra, or the dorsal count is miscoded.

If the skeletal reconstruction is accurate (and everything else wrong), the total length of the neck, trunk, and sacrum is 17 meters, so a total length of 30+ meters is pretty reasonable. But if everything else is right, the skeletal reconstruction is too big, by anywhere from 5-47%. I don’t think it’s really half again too big, but I’ll bet that the real number is lurking in there somewhere, probably around 15%.

So what’s the point? Why did I write the post?

Well, I certainly didn’t do it to make Calvo et al. look careless or stupid. The fact is, it is really, really easy to get scale bars wrong, and really, really hard to get skeletal reconstructions right. I have screwed up both (see Wedel 2000 for evidence). Big things are hard to measure, and it is perilously easy to get the numbers wrong. In the original paper on Sauroposeidon I (my coauthors are blameless here) wrote that it had 12-meter neck, the longest of any known animal. Turns out neither of those things is true. The most liberal neck estimate for Sauroposeidon is 11.5 meters, and the most conservative is only 10.5 meters. And Supersaurus had a longer neck, at least 13 or 14 meters no matter how you get there (Wedel and Cifelli 2005).

Here’s another example. Crack open just about any dinosaur book and you’ll read that Mamenchisaurus had a neck 33 feet long. But it ain’t the case. Tote up the lengths of the individual vertebrae in Young and Zhao (1972) and you’ll get 31.5 feet (9.5 meters). Stretch a tape measure along the mounted skeleton and you’ll get the same result. So where did the number 33 come from? Beats me. But a lot of people have repeated it, when the real numbers are not hard to get (at least for the people doing the repeating).

Also, I can’t fault Calvo et al. for publishing a short description of Futalognkosaurus, as long as they follow up with something more substantial later. It’s pretty common in biology (not just paleo), and it’s exactly what Rich and Kent and I did with Sauroposeidon. Describing things is a time-consuming process–four measly vertebrae were enough to keep me busy for years–and there are lots of reasons why it may be good to get something out now and follow up with the monograph as time and circumstances allow (but do follow up).

I wrote the post for the same reason I do all this stuff–when it comes to sauropods and their vertebrae, my curiosity knows no bounds. As soon as I saw the Futalognkosaurus description I knew I’d eventually dig in and see how big the critter is supposed to be. And now I have an outlet for that kind of nerdosity, hence the post.

The moral of the post, if there is one, is that tables of measurements are usually harder to screw up than scaled figures (but by no means invulnerable to error). And that until a more complete description of Futalognkosaurus appears–hopefully with a table of measurements–it’s hard to say how big it was. Maybe it was bigger than Brachiosaurus. Maybe not.

January 11, 2008

A hat-tip to Paul Barrett, who’s reminded us that technically we’re not supposed to be using photographs of Natural History Museum specimens — at least, not without acknowledgement. Our apologies go the museum for having overlooked this: we’d like to remind you all that all photographs of specimens owned by the museum are also copyright the museum, and we’ll try to remember to make that point explicitly in all future posts that show NHM specimens.

For the same reason, unfortunately, we can’t sell you Xenposeidon shirts — to do that would require a marketing agreement with the museum, which is not really a direction we want to go in … as you can imagine. So with apologies to those of you who didn’t snap one up while they were available, we’ve removed all the links from this site. (Congratulations to Mike From Ottawa, who, so far as I can tell, is in fact the only person outside the three of us who had the good taste to buy a shirt. Note to NHM commercial staff: our markup on that sale was £0.00 (or $0.00 in US funds) which we will be happy to hand over if you wish :-)

Finally, since we do promise “all sauropod vertebrae, all the time”, I sign off with a photograph: and a historically significant one at that:

Image copyright the Natural History Musuem, since it’s the museum’s material.

What we have here are, in a sense, the first sauropod specimens ever: the caudal vertebrae, and associated chevrons, that are the type material of the first named sauropod species, Cetiosaurus brevis Owen 1842. From the Wealden, natch. The vertebrae, from the anterior part of the tail, are BMNH R2544­-2547, and are shown in anterior view; the chevrons are BMNH R2548-­2550. These elements may belong to the same species, and maybe even the same individual, as the holotype of Pelorosaurus conybeari Mantell 1950. If you care to wade through the taxonomic quagmire associated with this series, you can find a discussion on pp. 1559-1560 of Taylor and Naish (2007) (the Xenoposeidon paper) — a discussion which I am more than happy to state, for the record, Darren wrote.

Update — 1 April 2009 (but not an April Fool)

Cetiosaurus brevis is not after all the first validly named Cetiosaurus species, because we (Darren and I) followed Upchurch and Martin (2003) in conflating nomenclatural and taxonomic validity. According to a strict interpretation of ICZN rules, C. medius is the type species, but we now have an ICZN petition out that should change that and fix C. oxoniensis as the type species, which is what everyone means in practice anyway.

References

Mantell, Gideon. A. 1850. On the Pelorosaurus: an undescribed gigantic terrestrial reptile, whose remains are associated with those of the Iguanodon and other saurians in the strata of Tilgate Forest, in Sussex. Philosophical Transactions of the Royal Society of London 140: 379-390.

Owen, Richard. 1842. Report on British fossil reptiles, Part II. Reports of the British Association for the Advancement of Science 11: 60-204.

January 8, 2008

A quick follow-up on Darren’s recent post: the reconstructed Argentinosaurus dorsal in the photo he used seems to be based on the anterior dorsal of the holotype — at the least, the proportions and most of the features are the same — so we can get some more information by looking at the figure of the same element in the description of Bonaparte and Coria (1993). Here it is:

This shows the details better than the photo, though admittedly it’s rather less spectacular. But what’s most noticable, to me at least, is that the centrum and the lower part of the neural arch is completely missing … which means that the “hypantrum” of the reconstructed vertebra in the photo that Darren used is pretty much a complete fiction. Or let’s be more charitable and say “involves a certain amount of interpretation”.

So! What’s the story? The dorsal figured here is the one that Bonaparte and Coria considered to be ?first, but the ?second and ?third (and more posterior dorsals) are also preserved — and more fully, at least in the relevant area. The ?second dorsal, shown in their figure 3, is not illustrated in anterior view, but the ?third is shown in their figure 4:

So now we can see what we came here for: the distinctly ventrolaterally sloping borders of the triangular hollow below the prezygapophyses. But is it a hypantrum? Well, the purpose of a hypantrum is to accept the hyposphene of the vertebra in front: but if this vertebra’s alleged hypantrum were filled by a hyposphene, its neural canal would be almost completely blocked off. So maybe not. And indeed the ?second dorsal of the Argentinosaurus type specimen, figured by B&C93 in posterior view as fig. 3A, doesn’t seem to have anything resembling a hyposphene. So that supports Salgado and Martínez’s (1993) assertion that the “hyposphenes” of Argentinosaurus are in fact just big old centropostzygapophyseal laminae. (Salgado and Bonaparte 2007 cited and reaffirmed this reidentification, but noted that the posterior dorsals of Big-A do seem to have big hyposphenes and hypantra. Unfortunately these don’t seem to be figured in any of the papers we’ve cited here, so who can say?)

At this point, I am going to stop (A) posting all the figures from Bonaparte and Coria 1993, and (B) pontificating … at least until I’ve read Apesteguía (2005) which — for shame — I haven’t, yet. Well, it’s a truly frightening piece of work.